1. Field of the Invention
This invention relates to a unit injector for an internal combustion engine, especially to a unit injector for a diesel engine.
2. Prior Art
As a fuel injection system for a diesel engine, a unit injector integrally composed of a pressurizer unit, functioning as a fuel injection pump, and a fuel injection nozzle unit is well-known. For example, Japan Published Unexamined patent application sho No. 57-28863 discloses a unit injector in which a spill port having a spill valve is formed in a conduit between a pressurized unit and a fuel injection nozzle unit, the spill port communicating with a return path.
The spill valve is driven by oil pressure developed in a pressure source to selectively open and shut the spill port for respectively connecting and disconnecting the conduit and the return path. This oil pressure to the spill valve is controlled by either a mechanical rotary slide valve or a solenoid valve. When the spill valve is closed, fuel is supplied from the pressurizer unit to the fuel injection nozzle unit. When the spill valve is opened, fuel from the pressurizer unit is bypassed through the spill port to the return path.
The above-mentioned prior art unit has a problem in that both the mechanical rotary slide valve and the solenoid valve have relatively slow responses, leading to inaccurate fuel injection. For improved responsiveness, an actuator having a piezoelectric element may be used to drive the spill valve. However, the change in length (i.e., effective stroke) obtainable from a piezoelectric element is element only about a thousandth of the total length of the element, so a large-sized actuator having a large element is required to obtain a desired length change. Such a large unit is difficult to install as an integrall part of a unit injector.